Current limiting fuse



Sept. 3, 1968 HOWARD ET AL 3,400,235

CURRENT LIMITING FUSE Filed Aug. 21, 1967 United States Patent 79,048 5Claims. (Cl. 337229) ABSTRACT OF THE DISCLOSURE A current limiting fusewhose fusible element is a strip of silver having along its lengthalternate straight portions and semi-circular bowed portions standing onthe same side of the strip, the bowed portions each containing tworelatively large apertures located on the longitudinal axis of the stripnear the adjoining straight portions, the elements being under lighttension and embedded throughout its length in granular arc extinguishingmaterial.

This invention relates to high voltage fuses of the type adapted tolimit the current when interrupting heavy overloads or short circuits.

High voltage fuses of the kind to which this invention applies are wellknown. Basically, this fuse consists of the following: a tubular casingmade of an insulating material; a number of relatively long fusiblestrips which extend lengthwise in the casing and are supported thereinspaced from one another and from the casing; a pair of metallicterminals which close the ends of the casing and are connected to theends of the fusible strips; and a granular arc extinguishing materialwhich completely fills the casing and embeds the fusible strips. In thisparticular construction, the fusible elements were straight strips of afusible metal such as silver placed in light tension between theterminals. Changes in the current loading subjected these fuses tothermal cycling, resulting in repeated expansion and contraction of thefusible strips, and this, in turn, frequently led to premature failureof the strips from mechanical fatigue. Thermal cycling is particularlysevere in fuses used to protect large motors because of the wide rangeof currents possible between starting, running and stopping.Consequently, the failure of fuses in motor circuits has beenparticularly high due to the extreme conditions leading to thermalcycling.

Fuses of this type are very often used to protect large motors which aresubjected to abrupt changes in loading, sudden overloads and highstarting currents. High currents, and in particular sudden changesthereof, subject the fusible elements to severe shock forces leading tocyclic stresses in the elements. Hence, fusible elements may fail fromfatigue due to cyclic mechanical stresses as well as thermal cycling, orusually from a combination of the two.

The object of this invention is to provide a current limiting fuse ofthe foregoing type in which the fusible elements are resistant tofatigue failure.

According to the invention the fuse has its fusible elements constructedsuch that they readily yield longitudinally in response to cyclic forcesor expansion and contraction from thermal cycling. Each element consistsof a relatively thin, narrow and long strip of a fusible metal such assilver having along its length alternate straight portions and bowedportions standing on the same side of the strip, the structure beingformed by bending short lengths of the strip to one side of the strip atspaced in tervals along its length. In a preferred form of the strip,each bowed portion is semi-circular in profile and con- 3,400,235Patented Sept. 3,, 1968 tains two relatively large apertures located onthe longitudinal axis of the strip near the adjoining straight portions.These apertures define regions of reduced cross section for initiatingfusion of the strip. The elements are similarly arranged in the casingunder light tension between supporting structures at the ends of thecasing, which structure also spaces the element one from another andfrom the casing, and they are embedded in a granular arc extinguishingmaterial which completely fills the casing.

A better understanding of the invention may be had from the descriptionto follow taken with the accompanying drawing of a fuse constructedaccording to the invention.

In the drawing, which is a cutaway perspective view of an embodiment ofthe invention, there is shown a high voltage current limiting fuse 10suitable for use in the protection of large electric motors and otherloads of this kind. Fuse 10 has a tubular casing 11, a pair of endterminals 12 and 13, a number of fusible elements of which two are shownat 14 and 15, an indicating element 16, means 17 and 18 for supportingelements 14, 15 in spaced relation to one another and to the casing, agranular arc extinguishing material completely filling the enclosuredefined by casing 11 and terminals 12, 13 and a device 20 for indicatingthat the fuse has blown. Casing 11 is a relatively long tube made of astrong, hard, heat resistant, electrical insulating material such as matof glass fibers bonded together with a polyester resin. Terminal 12 hasa relatively long skirt 21 which is an interference fit over the upperend of the casing, an inwardly formed shoulder 22, a short neck 23, andan outwardly flared end 24 closed by a cap 25 which has its outer edgerolled over the flared end and is adapted for supporting indicatingdevice 20. The terminal, including its end cap, is made of a conductivemetal such as copper and the regions where it makes contact with thefuse holder may be silver plated to ensure that a good electricalconnection is made between the terminal and bolder. Terminal 13 isexactly the same as terminal 12 except that its end cap 26 is plain, andit is pressed onto the lower end of the casing in the same way thatterminal 12 is pressed onto the upper end of the casing.

Fusible elements 14 and 15 are relatively thin, narrow and long stripsof a fusible metal such as silver. They are two of a large number whichcan be used, depending on the current rating of the fuse. In the fuseillustrated up to twelve strips such as 14, 15 can be placed in parallelbetween terminals 12 and 13. These strips are supported inside thecasing in an annular array parallel with the casing, spaced therefromand from one another by means of support means 17 and 18 gripping therespective ends of the strips. An end portion of each strip extendsabove support means 17 in between the flared end 24 of terminal 12 andits end cap 25 where the ends are firmly secured to the terminalstructure, these ends having been brought out over the flared terminalend before the cap is applied. Since the ends of the strips are squeezedbetween the flared surface and the rolled edge of the cap, a goodelectrical connection is, therefore, made between the ends of the stripsand the terminal. The other ends of the strips extend below supportmeans 18 and are secured to terminal 13 in exactly the same way.

Means 17 and 18 for supporting the strips 14, 15 in spaced relation aretwo identical discs of an insulating material. Disc 17 is fitted insidethe skirt of terminal 12 at the upper end of casing 11, and is securedto these components by having its outer edge wedged between the end ofthe casing and the shoulder 22 on the terminal. Disc 18 is secured tothe lower end of the casing and terminal 13 in exactly the same way.Each disc, as particularly indicated for disc 17, has a large centralopening 27 and a plurality of slits 28 radiating from the periphery ofthe opening at equally spaced intervals. The width of the slit is alittle greater than the thickness of the strip and its depth about equalto the width of the strip. Hence the strip fits freely into the slitwhere it is held with its flat sides radial with respect to the tubularcasing. Each strip is also held under light tension between discs 17, 18by means of a pair of dents 29 and 30 in the strip resting against theouter surfaces of the discs. The slits in the two discs are alignedaxially of the casing so the strips are parallel with the axis of thecasing.

Current limiting fuses of this type invariably include a fusible elementsuch as 16 which is of much higher resistance and fusing temperaturethan elements such as 14, 15. This element introduces resistance intothe circuit as elements 14, 15 blow, and immediately after all theelements 14, 15 have blown, it too blows and triggers the indicatingdevice 20 into displaying a flag which gives a visual indication thatthe fuse has blown. This feature is well known in the art and isapplicable to fuses using many kinds, of fusible elements in place of 14and 15.

The granular arc extinguishing material may be a sand well known in theart for its arc quenching properties. The fuse is completely assembledexcept for cover 26 before it is filled with the sand. Then apply thesand, the fuse is placed upright with its closed end down, the reverseof that illustrated, and vibrated while dry sand is poured into itthrough its open end. After all the spaces inside the enclosure definedby casing 11 and terminals 12 and 13 are completely full of sand and thefusible elements well embedded therein, cover 26 is applied to completethe assembly.

The novel fusible elements 14 and 15 will now be described. As alreadypointed out, there is usually more than just the two elements identifiedat 14 and 15. However, as they are all alike no matter how many areused, only one, that identified by numeral 14, will be described. Aswell as being alike, the elements are placed in a like manner inside thecasing so that the parallel circuit formed by the elements is balancedboth electrically and mechanically.

Fusible element 14 is a relatively thin, narrow and long strip of afusible metal such as silver having along its length between supportmeans 17 and 18 straight portions 31, 32, 33, 34, etc., alternating withbowed portions 41, 42, 43, 44, etc. The straight portions lie in theoriginal plane of the flat strip and the bowed portions are bent out ofthe flat strip to one side of the plane. Preferably, the straightportions are relatively short and the bowed portions much longer andsemi-circular in profile, appearing along the edge of the strip as aseries of humps spaced apart along one side of the strip. Moreover, thestraight portions are all alike, appearing rectangular in plan form, andthe bowed portions are also all alike and uniform across the strip. Byhaving a uniform arrangement of straight and bowed portions, the stripis rendered uniformly resilient throughout its supported length, thusenabling it to yield uniformly as the structure changes temperature. Ithas already been pointed out that the strips are stretched a littlebetween their support means so as to place them under light tension.They are also arranged similarly in the casing, that is, the humps onone strip point into the hollows of the next adjacent strip. By lightlystretching the strip, its straight portions 31, 32, 33, 34, etc., areheld in alignment so that the bowed portions of one strip are located ina definite relation to the bowed portions of the adjacent strips. Thestraight portions serve another very useful purpose. They hold thestrips in their proper relation while the fuse is filled with sand. Itis very important that the strips be not distorted by the sand becausethis can seriously impair the rupturing capabilities of the fuse.

In order to initiate fusion of the strip at specific points along itslength, it is customary to reduce the cross section of the strip atthese points. When the fuse now blows, the

l V J 3,400,235

4 strip ruptures at different points, enabling the sand to quench theseries of arcs which follow. In the case of a strip such as 14, goodrupture characteristics have been obtained by reducing the cross sectionof each bowed portion in tWo places, a reduction near its juncture witheach of the two adjoining straight portions. The reductions in crosssection illustrated consist of a pair of apertures in each bowed portionnear the beginning and end thereof, for example, apertures 51 and 52 inbowed portion 41, apertures 53 and 54 in bowed portion 42, apertures 55and 56 in bowed portion 43, etc. These apertures are relatively large,of the same size and shape, and are located in the middle of the stripso the two edge portions left are as closely as possible to being equalin cross section.

Fusible elements of the configuration illustrated at 14 and applied in astructure of the kind described are found to have good rupturecharacteristics and resistance to fatigue. An alternate arrangement ofstraight and bowed portions such as this leads to a fairly equaldistribution of the flexing of a strip throughout its length, therebypreventing concentrations of flexure which eventually cause failure ofthe strip from fatigue. Uniform flexing among the bowed portions keepsthe stress intensity well within the endurance range of the metal at allpoints along the length of the strip. When the fuse blows, the series ofrestricted cross sections fuse to provide a series of arcs, the sum ofwhich produces a high are voltage. This fuse is well suited for use inmotor circuits in that it can accommodate the high voltages associatedwith motor starting and the varying current conditions to which themotor is subjected.

While there has been shown and described a particular embodiment of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andtherefore it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What we claim as new and desire to secure by Letters Patent is:

1. A fuse having a tubular casing, a terminal structure closing each endof the casing, a plurality of elongated fusible elements inside saidcasing extending lengthwise thereof from one terminal structure to theother, means located at the ends of said casing for retaining saidelements in light tension and in said spaced relation to one another andto the casing, and a granular arc extinguishing material filling thecasing and embedding the elements said elements being alike, similarlylocated in the casing, and each one comprising a fiat, relatively thin,narrow and long strip of a fusible conducting material having along itslength between said retaining means straight portions lying in the planeof the original flat strip alternating with bowed portions lying on thesame side of said plane and appearing in edge profile as a series ofhumps extending across the strip at spaced intervals along its length,said straight portions being relatively short in length and said bowedportions being considerably longer lengths bent out of the strip to oneside thereof, said alternate straight and bowed portions impartinguniform resilience to the strip for the accommodation of thermal cyclingand cyclic stressing of the element to overcome failure thereof fromfatigue.

2. The fuse defined in claim 1 wherein the means for retaining thefusible strips in light tension and in spaced relation comprises thefollowing: two annular discs located one at each end of the casing andsecured thereto along the outer periphery of the disc, each one of saiddiscs having a plurality of slits equally spaced around its innerperiphery and extending radially outward therefrom a distanceapproximately equal to the width of the fusible strip; each one of saidstrips having its ends located in a corresponding one of the slits inthe two discs; two dents in each strip near the ends thereof bearingagainst the 5 outer surfaces of the discs from the light tension in thestrip.

3. The fuse defined in claim 1 wherein each strip is reduced in crosssection at spaced intervals between said retaining means.

4. The fuse defined in claim 1 wherein each one of said bowed portionscontains two relatively large apertures located in the middle of thestrip near the adjoining straight portions.

5. The fuse defined in claim 1 wherein each one of said straightportions contains a relatively large aperture located in the middle ofthe portion.

References Cited UNITED STATES PATENTS 2,734,111 2/1956 Kozacka 200-1203,080,463 3/1963 Kozacka 200 -120 2,988,620 6/1961 Kozacka 200l35 XRBERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner.

